The present invention relates to dispersions of biodegradable polymers. The invention particularly concerns a process for the preparation of a hydrophobic polymer dispersion characterized by a) forming a mixture of a biopolymer, a plasticizer, dispersion auxiliaries and water, b) heating up the mixture in order to obtain a paste-like composition, and c) diluting the paste-like composition in water.
The current emphasis on an environmentally friendly attitude and green values is opening up new markets for products based on renewable natural resources. Such trends can be observed within the sectors of, e.g. the packaging industry, the sanitary industry and the glue industry, where recyclability, reuse, compostability, biodegradability and lack of environmental strain are demands of today. The trend of replacing products based on petrochemistry by processed biopolymer products is also emphasized. Starch and its derivatives constitute a particularly interesting initial material for the production of biodegradable polymer products. Other important biodegradable or compostable polymers include, among others, polycaprolactone, polylactide and other lactic acid based polymers and copolymers, such as polyester urethanes.
Polymer dispersions are used, e.g., in the coating of paper and board in order to improve their water resistance. Dispersions used in the paper industry today contain, in addition to fillers, various synthetic latexes which are quite poorly degraded in a biological environment.
Solutions are previously known where modified starch components have been used for the preparation of paper coating dispersions. Thus, a hydrophobic coating composition is known from WO Published Application No. 93/11300 which is based on a polymer dispersion containing a starch derivative having a polymer containing styrene and butadiene monomers grafted thereto. Thus, this solution does not use any of the biodegradable components which would be compatible with the demands for recyclability made on the products.
The present invention aims at achieving a hydrophobic dispersion whereby at least most and preferably essentially all of its components are biodegradable. The invention especially aims at providing a dispersion whose polymer component mainly comprises a biodegradable polymer (below also termed xe2x80x9cbiopolymerxe2x80x9d), advantageously starch or a derivative thereof and/or lactic acid based polymers and copolymers. The invention further aims at providing a process for the preparation of hydrophobic dispersions.
The invention is based on the surprising observation that stable dispersions can be prepared from many biodegradable polymers by forming a paste-like composition of the polymer in question, a plasticizer, dispersion auxiliaries and water, which composition is dispersed in water. No solvents are then required for preparing the dispersion. Thus, according to the present process
the polymer used is a biodegradable polymer which is admixed with a plasticizer, auxiliaries and water in order to form a premixture,
the premixture is heated in order to provide a paste-like composition, and
the paste-like composition is diluted/dispersed in water.
In more detail, the method according to the invention is characterized by a process for the preparation of a hydrophobic polymer dispersion, characterized by a) forming a mixture of a biopolymer, a plasticizer, dispersion auxiliaries and water, b) heating up the mixture in order to obtain a paste-like composition, and c) diluting the paste-like composition in water.
The invention offers considerable benefits. Thus, the raw materials of the polymer dispersion according to the present invention are mainly based on renewable natural resources and are biodegradable/compostable. The method is particularly well suited for finely divided/powdery polymers. The starch component may be derived from any native starch; it need not be, e.g., a starch rich in amylose. No solvents requiring removal by evaporation need be used for formulating the dispersion, instead, the dispersing may be performed in a conventional disperser or even a mixer. There is no need to dry the polymer prior to dispersing.
Films made from the dispersion are quite resistant to water and can be exploited to greatly improve the water resistance of paper or board. The dispersions also have good adherence properties. Thus, the novel polymer dispersions can be used for coating paper or board, as primers, as adhesive component in water-soluble adhesives (e.g. labelling adhesives) or as a component in paint or lacquer. The examples 10 and 11 deal with the use of the dispersions for improving the water resistance of paper coating latexes. As will emerge from the examples the dispersions can be used for partly or entirely replacing synthetic binders (e.g. 10 to 90% of a synthetic binder, 90 to 10% of the present dispersion).
The suitability of the dispersions for use as lamination adhesives is examined in Example 5 and it will emerge that a plastic film laminated onto paperboard using the dispersion could not possibly be detached without breaking the board surface. Example 7, then, assesses the drying properties of the dispersions when used as paint vehicle, and it is found that films made from the dispersions are plastic-like and dry quickly. Therefore, the dispersions are excellently suited for use as paint vehicles. It has also been found that the dispersions can be used as lacquers for the surface treatment of wood and other materials. They are also suitable in the production of hydrophobic cast films and as binders in materials based on cellulose fibres. The water resistance of the dispersions can be further improved by modifying them using cross-linking chemicals. If desired, these can be combined with the use of wax.
In the following the present invention is examined in greater detail by means of a detailed description and a number of working examples.
According to the invention, dispersions of biodegradable polymers are advantageously prepared in three steps whereafter the dispersions can be treated further using modifying chemicals in order to achieve e.g. improved water resistance. If required, the dispersions can also be homogenized.
First, a mixture is formed of the biopolymer, dispersion auxiliaries (including protective colloids, if any) and water. Depending on the biopolymer, the mixture may also contain, for example, a polymer plasticizer and other adjuvants and auxiliaries. The mixture contains a part of the total amount of water in the dispersion. Advantageously, 10 to 90%, preferably about 20 to 80%, of the total amount of water in the final dispersion is contained in the mixture, whereby the premixture contains advantageously 1 to 200 parts by weight, preferably about 50 to 150 parts by weight of water, to 100 parts by weight of the polymer. The aim is to provide a sufficiently stable and viscous mixture. The mixture is stirred to provide a homogeneous mass by applying intensive stirring.
The dispersion auxiliaries are most advantageously dissolved or mixed into the water to be added into the premixture before being intermixed with the polymer.
During the second step of the process the mixture is heated to about 20 to 100xc2x0 C., preferably about 40 to 60xc2x0 C. The temperature varies according to the polymer used and the composition of the dispersion. Stirring is continued at the temperature in question until a suitably paste-like mixture results. In the case of certain polymers no actual heating step is required and the paste-like mixture is instead obtained by continuing the stirring at room temperature.
The stirring times are usually equally divided between the formation of the mixture and the heating step. Depending on the amounts of the materials, the mixture is stirred for about 1 min to 24 h, advantageously about 10 min to 2 h, during the first step of the process, and for about 1 min to 24 h, advantageously about 10 min to 2 h, during the second step of the process. The total stirring time is typically between about 5 min and 4 h.
During a third step of the process the rest of the calculated amount of water in the dispersion is added to the paste-like mixture. The addition is advantageously performed gradually and under simultaneous intensive stirring of the dispersion. According to a preferred embodiment of the invention water is added to the paste at an elevated temperature ( greater than 20xc2x0 C.), whereby it has been found of particular advantage to add the water to hot paste while simultaneously allowing the paste to cool. However, it is also possible to dilute the paste in water at room temperature or even at lower temperatures. Water is most advantageously added at such a rate that the plasticized mass remains homogeneous and no separation of phases occurs. During the addition, the vaporizing water is condensed and recycled to the mixture.
Even though the above describes how a dispersion is formed in water, even different kinds of solvents can be used as the dispersion medium.
According to a particularly preferred embodiment the dispersion is prepared such that
100 parts by weight of a starch ester, 5 to 50 parts by weight of a protective colloid and 10 to 200 parts by weight of a plasticizer are admixed with 70 to 120 parts by weight of water containing 0.1 to 10 parts by weight of a surface-active agent in order to provide a starch ester blend,
after initial stirring, the temperature of the blend is raised to 40 to 90xc2x0 C. and the stirring is continued until a white paste-like composition is obtained, and
50 to 150 parts by weight of water are added to said composition whilst allowing the temperature to decline to room temperature.
After dispersing the dispersion is, if desired, homogenized in a manner known as such in order to stabilize it. The homogenization may be achieved e.g. by means of a pressure homogenizer. The homogenization may be applied to reduce particle size by 50 to 100%, thereby further improving the stability of the dispersion. Dispersions according to the invention will remain stable for several weeks, even months.
As stated above, the water resistance of the dispersions may be further improved by means of cross-linking chemicals. These include, among others, glycosal and aluminium and zirconium salts. Of aluminium salts, aluminium acetyl acetonate, in particular, may be mentioned. The cross-linking chemicals are normally used in an amount of about 0.01 to 30%, advantageously about 0.1 to 10%, and preferably about 0.5 to 8%, of the weight of the dispersion.
The effect of cross-linking chemicals on the water resistance of the dispersions and, correspondingly, on the water resistance of board coated with the dispersion, is examined in Examples 6 and 8.
Hydrophobic starch dispersions may be prepared in the above-described manner containing as their starch component a starch ester, starch ether, mixed ester/ether of starch or grafted starch made from native starch, hydrolyzed starch, oxidized starch, cross-linked starch or gelatinized starch. Hydrophobic polymer dispersions may also be prepared from other biodegradable polymers, such as polycaprolactone, lactic acid polymers, lactic acid-based polyester urethanes, polylactide, cellulose acetate and/or polyhydroxy butyrate/valerate. The last-mentioned polymers may of course be used in mixtures with the starch polymers. It is particularly advantageous to provide hydrophobic polymer dispersions containing starch ester and hydroxy alkyl starch ester. The process is especially well suited for powdery base materials.
In the composition according to the invention starch or a derivative thereof, in the following also termed starch component, may be based on any native starch having an amylose content of 0 to 100% and an amylopectin content of 100 to 0%. Thus the starch component may be derived from barley, potato, wheat, oats, peas, corn, tapioca, sago, rice or any similar tuber-bearing or grain plant. It can also be based on starches prepared from said native starches by oxidizing, hydrolyzing, cross-linking, cationizing, grafting, or by etherification or esterification.
It has been found advantageous to use a starch-based component derived from an ester formed by starch and one or more aliphatic C2-24 carboxyl acids. The carboxyl acid component of such an ester may then be derived from a lower alkane acid such as acetic acid, propionic acid or butyric acid or a mixture thereof The carboxyl acid component may, however, also be derived from a saturated or an unsaturated natural fatty acid. Examples of these include palmitinic acid, stearic acid, oleic acid, linoleic acid and mixtures thereof. The ester may also be composed of both long- and short-chain carboxyl acid components. As an example, a mixed ester of acetate and stearate may be mentioned. In addition to acids, the corresponding acid anhydrides and acid chlorides and other corresponding reactive acid derivatives may also be used in a manner known as such for forming the ester.
The preparation of the fatty acid esters of starch is performed, for instance, in the manner described in the following publications relevant in the field: Wolff, I. A., Olds, D. W. and Hilbert, G. E., The Acylation of Corn Starch, Amylose and Amylopectin, J. Amer. Chem. Soc. 73 (1952) 346-349, or Gros, A. T. and Feuge, R. O., Properties of Fatty Acid Esters of Amylose, J. Amer. Oil Chemists"" Soc 39 (1962) 19-24.
Starch acetates may be prepared by allowing the starch to react with acetanhydride in the presence of a catalyst. As the catalyst, for example a 50% sodium hydroxide is used. Even other known methods described in the literature for preparing acetates are suited for the preparation of a starch acetate. By varying the amount of acetic acid anhydride, the amount of the base used as the catalyst, and the reaction time, starch acetates having different degrees of substitution can be prepared.
According to a preferred embodiment the starch component is an esterified starch, advantageously a starch acetate having a degree of substitution between 0.5 and 3, advantageously between 1.5 and 3 and preferably between 2 and 3. It is particularly preferred to use, e.g., enzymatically hydrolyzed barley starch for the preparation of the starch esters.
According to another preferred embodiment the starch component comprises hydroxyalkylated starch or an ester thereof. Hereby it is particularly preferred to use hydroxy propyl starch having a molar degree of substitution of 1.4 at the most, advantageously 1 at the most, and hydroxy propyl starch esters having a molar degree of substitution of 1.4 at the most, advantageously 1 at the most and preferably 0.1 to 0.8, and the degree of substitution is at least 2, advantageously 2.5 to 3.
When the dispersions are prepared from starch or derivatives thereof the dispersion composition is advantageously also made to contain a plasticizer which is advantageously present in an amount of 0.01 to 95% by weight, preferably about 1 to 50% by weight of a plasticizer. Any known plasticizers can be used, examples thereof including the following: triacetin, diacetin, monoacetin, triethyl citrate, tributyl citrate, acetyl triethyl citrate, acetyl tributyl citrate, dimethyl succinate, diethyl succinate, oligoesters of amber acid and diols, ethyl lactate, methyl lactate, fatty acid esters of glycerol, castor oil, olive oil, rapeseed oil, tall oil, dibutyl phthalate, diethyl phthalate, and mixtures thereof.
The starch dispersion further contains a dispersion auxiliary by means of which the biopolymer can be dispersed in water in sufficiently finely divided particles in order to form a stable dispersion. The dispersion auxiliaries comprise surface-active agents and protective colloids. Examples of the surface-active agents include cationic, anionic and non-ionic tensides, such as polyoxyethylene derivatives (polysorbates, polyoxyethylene alcohols), saponin, alkyl sulphonates, alkyl benzene sulphonates, fatty acid soap; lignosulphonates, sarcosinates, fatty acid amines and amides, phospholipids, sugar lipids, lecithin and/or wood extracts. The surface-active agents are usually present in an amount of about 0.1 to 20%, preferably about 0.5 to 15%, of the weight of the biopolymer.
Suitable protective colloids include polyvinyl alcohol (PVA), particularly PVA having a weight-average molar mass of about 10.000 to 115.000, and cationic starch and hydroxy alkyl starch as well as hydroxy alkyl cellulose, which may be used separately or together with PVA. The dispersions may further contain alkyl ethene dimer wax (AKD), beeswax or carnauba wax as adjuvants or auxiliaries. These are typically present in an amount of about 1 to 80%, preferably about 5 to 60%, of the weight of the polymer.
To cite an example of advantageous polymer dispersion compositions, a composition may be mentioned containing
5 to 25 parts by weight of a starch ester,
5 to 50 parts by weight of a plasticizer,
1 to 150 parts by weight of water and
1 to 20 parts by weight of a dispersion auxiliary.
Water is typically present in an amount which is approximately 2- . . . 10, and advantageously about 4- . . . 6 times the amount of the biodegradable polymer, plasticizer in an amount which is about 0.1- . . . 2 times and auxiliary in an amount which is about 0.1-1.5 times the amount of the biodegradable polymer. Thus, by way of exemplifying the preparation of a hydrophobic starch ester dispersion, about 1.0 to 5 kg of starch ester, 0.5 to 6 kg of plasticizer and about 0.1 to 1.5 kg of auxiliary may be dispersed in 10 kg of water.
Depending on the intended use, such a polymer dispersion may further be made to contain 0.01 to 30% by weight, preferably about 5 to 30% by weight of a cellulose ester, such as cellulose acetate, cellulose propionate or cellulose butyrate, or mixed esters thereof.
The dispersions may further contain adjuvants and/or auxiliaries known within the plastics technology as such. Tall resins and derivatives thereof may be used to improve the adhesion properties of the dispersions. Because tall resins contain double bonds they also serve to improve the drying properties of the dispersions. Tall resins are typically present in an amount of 1 to 50%, advantageously about 5 to 30%, of the weight of the biopolymer. In paint applications the dispersions may contain 0.1 to 80% of alkyd compounds (binders commonly used in paints and printing inks) of the amount of starch or a corresponding biopolymer. These are typically condensate products formed of a bivalent acid, polyol (glycerol or pentathritol) and a fatty acid and natural oil. For example, the fatty acid ester of pentaerythritol may be cited which is also used in Example 1 (Table 1, test 13).
The invention provides dispersions of biodegradable polymers, in particular starch derivatives, preferably starch esters, wherein 90% of the polymer particles are smaller than 10 xcexcm. By homogenization dispersions are obtained having particle sizes which on the average are below 2 xcexcm or even below 1 xcexcm. Depending on the intended use, dispersions may further be prepared having a multimodal, in practice mostly a bimodal particle size distribution.
The following non-limiting examples are provided by way of illustrating the invention. The substitution degrees of the starch acetates cited in the examples are determined in accordance with Wurzburg (Wurzburg, O. B, Acetylation, in: Methods in Carbohydrate Chemistry, Vol. IV, ed. R. L. Whistler, Academic Press, New York and London, 1964, p. 288). The molar masses, then, have been determined on the basis of GPC analysis in the Alcohol Inspection Laboratory. The equipment used was HP-1090, two in a column series (Waters, Ultra Hydrogel 2000), solvent 50 nM NaOH, temperature 40xc2x0 C., dextran standards, as detectors RI and viscosity detectors. The molar mass has been determined from the starch used as the starting material.